Field of the Invention
The field of art to which this invention relates includes well tools, and more particularly tools for use in running, setting and testing packoff assemblies in oil or gas wells. United States patents on this type of apparatus can be found in class 166 of the United States Patent Classification System.
Prior to the present invention, various types of devices called packoffs have been used for sealing the annulus between two concentric well elements, such as a wellhead and an inner casing hanger. One well-known type of packoff has an annular resilient seal assembly with a smalller inner diameter and a larger outer diameter than that of the annulus in which it is designed to function, so that when this packoff is set in the annulus an interference seal sufficient to hold the anticipated well pressure is established with the adjacent well elements. Although it has some desirable features, this type of packoff has a serious drawback in that its seal is easily damaged, especially while lowering the packoff through a close-fitting blowout preventer. When such damage occurs the seal must be replaced, since there is no provision for energizing the seal or otherwise overcoming the damage.
Another common type of packoff is set by utilizing the weight of its running string to axially compress the seal element and force it to expand radially into tight contact with the adjacent well elements, and then locking the packoff in this condition with hold-down devices called slips. One of the problems with this "weight-set" packoff is that the slips are difficult to release when it is desired to retrieve the packoff from the well. Another problem is that the tapered surfaces of the slips, which establish a friction lock between the packoff and the adjacent well elements, significantly limit the number of locations at which this type of packoff can be set.
A variation of the aforementioned "weight-set" packoff involves the use of hydraulic pressure to produce the downward force needed to compress the seal element sufficiently to establish an adequate pressure barrier. Since this "hydraulic-set" packoff also is locked in place by slips, it is plagued by the same disadvantages of the " weight-set" device.
The most commonly used type of packoff is run into the well on a drill string, and then set by rotating the string to exert torque on the packoff's seal-compressing ring, thereby forcing the seal radially into pressure-tight contact with the adjacent well elements. One of the most desirable advantages of this "torque-set" packoff is that after it has been set in the well the pressure-holding capability of its seal element can be increased by torquing it further, i.e., by further rotation of the drill string. However, when torque-set packoffs are employed at offshore locations where the wellhead is a significant distance below the drilling platform, such as on the ocean floor in deep water, a considerable amount of friction drag exists between the drill string and the walls of the riser, thus requiring the application of excessive torque on the string to fully set the packoff, and incurring the likelihood of serious damage to the string. Furthermore, the vertical movement of the drilling rig on an offshore floating platform makes it very difficult to maintain adequate control of the drill string while setting the packoff, especially during rough weather and heavy seas as, for example, are frequently experienced in the North Sea.